Lincoln Electric SVM 106-A service manual SCR Operation, Ground Lead Protection

THEORY OF OPERATION E-7

Return to Section TOC

Return to Section TOC

Section TOC

Return to Master TOC

Return to Master TOC

Master TOC

GROUND LEAD PROTECTION

The Ground Lead Protector (GLP) circuit protects the ground lead that connects the wire feeder to the welding power source. When the circuit detects any current flow in the ground lead, a contact closes in the control circuit which de-energizes the control relay. Refer to Figure E.5. This opens the circuit for the welding relay in the welding power supply to remove power from the welding gun, shuts the gas and water solenoids, and stops the wire feed motor. The GLP circuit remains in this tripped state until the reset button on the wire feeder is pressed, or power is removed from the wire feeder.

SCR OPERATION

Since the wire speed of the LN-7GMA is controlled by SCR circuitry, a basic understanding of how an SCR functions is important. Refer to Figure E.6 and the following explanation. A silicon controlled rectifier (SCR) is a three terminal semi- conductor device used to control currents to a load. An SCR acts very much like a switch. When a gate signal is applied to the SCR it is turned on and there is current flow from anode to cathode. In the on state the SCR acts like a closed switch. When the SCR is turned off there is no current flow, thus the device acts like an open switch. As the name

suggests, the SCR is a rectifier, so it passes current only during positive half cycles of the AC supply, The positive half cycle is the portion of the sine wave in which the anode of the SCR is more positive than the cathode.

When an AC supply voltage is applied to the SCR, the device spends a certain portion of the AC cycle time in the on state and the remainder of the time in the off state. The amount of time spent in the on state is controlled by the gate.

An SCR is fired by a short burst of current into the gate. This gate pulse must be more positive than the cathode voltage. Since there is a standard pn junction between the gate and cathode, the voltage between these terminals must be slightly greater than

0.6VDC. Once the SCR has fired it is not necessary to continue the flow of gate current. As long as current continues to flow from anode to cathode the SCR will remain on. When the anode to cathode current drops below a minimum value, called holding current, the SCR will shut off. This normally occurs as the AC supply voltage passes through zero into the negative portion of the sine wave. If the SCR is turned on early in the positive half cycle, the conduction time is longer, resulting in greater SCR output. If the gate firing time is later in the cycle the conduction time is less, resulting in lower SCR output.

Return to

Return to

FIGURE E.6 – SILICON-CONTROLLED RECTIFIER OPERATION.

INPUT

Return to Section TOC

Return to Master TOC

ANODE

CATHODE OUTPUT

GATE

GATE

NOTE: AS THE GATE PULSE IS APPLIED LATER IN THE CYCLE THE SCR OUTPUT IS DECREASED.

LN-7 GMA WIRE FEEDER